This invention relates to compressors of air or other gases and in particular, but not exclusively, relates to compressors for the pharmaceutical and food industries in which compressed gases free from oil are required.
Such a compressor generally comprises a centrifugal impeller overhung at one end or an impeller overhung at each end of the shaft of the armature of a high frequency electric motor. The armature of the motor carries permanent magnets which become ineffective at temperatures above some characteristic value. A major problem in the design of compressors of that type is to arrange cooling so that the magnets are not degraded by a too severe rise in temperature. The problem of cooling is compounded because of the absence of lubricating oil which otherwise would convect away heat from a compressor to its oil cooler.
Two mechanisms of generation of heat have to be regarded. Although in principle no eddy currents are induced in the armature of the motor, in fact some eddy currents exist because of inevitable departures from perfection in the practical application of the electromagnetic principles of the motor. The first mechanism is the generation of heat by these trace eddy currents. The second mechanism is the generation of frictional heat at the journal bearings and at the thrust bearing.
It may seem appropriate to carry away heat from both of these sources by flows of air or of gas derived from the compressor itself. Although that is a feasible method for carrying heat away from the bearings it is found from heat transfer calculations to be inherently inadequate by itself for the cooling of the armature. At the armature the essential requirement is that the temperature of the permanent magnets should not exceed some characteristic value which in turn requires the heat generated by the trace eddy currents to flow from armature to coolant under the limited temperature difference dictated by the limiting temperature of the magnets.